In recent years, as semiconductor memory devices such as DRAM (Dynamic Random Access Memory) have been increasing in storage capacity with their higher densities and higher integrations, there have been studied semiconductor memories utilizing high dielectric thin film materials having higher dielectric constants as compared with silicon oxide. Among the high dielectric materials are STO (SrTiO.sub.3 ; strontium titanate), BST ((Ba,Sr)TiO.sub.3 ; barium-strontium titanate), tantalum oxide (Ta.sub.2 O.sub.5) and the like, which are under discussion for applications to high-integration DRAMs or the like.
On the other hand, ferroelectric materials having many functions such as pyroelectricity, piezoelectricity and electro-optical effect are applied to a wide variety of device development including infrared sensors, piezoelectric filters, optical modulators and the like. Among others, nonvolatile memory devices (ferroelectric memory devices) utilizing the unique electrical characteristic of spontaneous polarization have been widely studied in view of their potentiality of replacing most memories from conventional nonvolatile memories to SRAMs (static RAMs) and DRAMs by virtue of their fast write/read operations, low voltage operations and other characteristics.
The mainstream of ferroelectric materials has been those belonging to perovskite type oxides typified by PZT (Pb(Zr,Ti)O.sub.3 ; lead zirco-titanate). However, in recent years, bismuth layer-structured compound materials such as SrBi.sub.2 Ta.sub.2 O.sub.9 have been gaining attention in terms of their resistance to repetition of polarization inversion and being investigated for practical application to ferroelectric memory devices.
Generally, in a semiconductor memory device which uses the aforementioned oxide thin-film materials as a capacitor insulating layer, after the formation of an upper electrode, it is covered with an interlayer insulating film of BPSG (boro-phospho silicate glass) or the like which purposes primarily electrical insulation between semiconductor memory devices. In this case, unfortunately, hydrogen gas produced as a reactive byproduct has a reduction effect on the oxide thin-film interface so that the adhesion property between the upper electrode and the oxide thin film is lowered, which leads to a problem that peeling occurs between the upper electrode and the oxide thin film. There is a further problem that, under the influence of the hydrogen gas, the dielectric constant of the capacitor lowers, or in the case of a ferroelectric thin film, deterioration of its characteristics occurs. This has been a great obstacle in practicalizing devices using semiconductor memory devices in which the above oxide thin-film materials are employed as a capacitor insulating film.
Also, in a semiconductor memory device which uses MOS (Metal Oxide Semiconductor) transistors as a switching device, lattice defects occurring within a silicon single crystal substrate during the manufacturing process would cause characteristic deterioration of the MOS transistors. This would require restoring the MOS characteristics by heat treatment in a hydrogen-mixed nitrogen gas (forming gas) in the final process. However, the concentration of hydrogen in that process is higher than that of hydrogen produced during the formation of the aforementioned interlayer insulating film, thus having a very large effect on the capacitor.
In order to solve these problems, the following proposals have been made. First, in a ferroelectric memory described in Japanese Patent Laid-Open Publication HEI 7-111318, upper part of a capacitor is coated with an Al, Si or Ti nitride thin film, which serves for a protective film. However, this protective film would crystallize at firing temperatures for crystallization of SrBi.sub.2 Ta.sub.2 O.sub.9 when SrBi.sub.2 Ta.sub.2 O.sub.9 is used as the ferroelectric substance. Then, the crystallized protective film disadvantageously has difficulty in obtaining enough hydrogen-gas blocking property because grain boundaries serve as paths. This would occur likewise also when a crystal protective film such as TiN film is used.
Also, in a ferroelectric memory described in Japanese Patent Laid-Open Publication HEI 7-273297, a metal oxide layer which reacts with moisture content adsorbed to the inside of a ferroelectric thin film is used as a first protective film, and a ferroelectric layer which reacts with hydrogen gas produced in the process of forming an interlayer insulating film is used as a second protective film. However, when an insulator like the metal oxide that is the first protective film is used as the protective film for upper part of the capacitor, it is necessary to provide an opening as a takeout hole for the upper electrode, so that enough effect as a protective film could not be expected. Otherwise, because of lack of electrical conductivity, some structural contrivance would be necessitated, which leads to a problem that contrivances for film deposition or processing become also complex.
Further, in the case of a memory device having a structure that several kinds of electrodes and metal wirings are present with the protective film interposed therebetween, as in the second protective film, when the protective film itself comes to have ferroelectricity, there may occur trouble in the operation of the memory device. Therefore, it is necessary to suppress the development of ferroelectricity, for example, by non-crystallizing or partly non-crystallizing the protective film, which leads to another problem that the manufacturing process becomes complex.
In any case, the above protective films still remain problematic as a material forming the upper electrode.
Furthermore, when an oxide high dielectric such as Ta.sub.2 O.sub.5 is used as a capacitor insulating film for DRAMs or the like, TiN film is commonly used as the upper electrode. In this case, there is a problem that oxygen of the capacitor insulating film escapes to the upper electrode during the annealing after the formation of an interlayer insulating film, causing an increase of the leakage current.